High voltage switching transistors, such as power MOSFETs, junction field effect transistor (JFETs) and gallium nitride (GaN) high electron mobility transistor (HEMT), are commonly used as semiconductor switches in high voltage and high power devices such as switched-mode power supplies, motor controllers, and high voltage and high power switching circuits. Some of these devices, such as the GaN HEMT have the ability to be operated at very high voltages without the device breaking down or becoming damaged.
Some devices, such as the JFET and the GaN HEMT may be fabricated to have a negative threshold voltage, thereby causing the device to be conductive when zero voltage is across the gate and source of these transistors. Such devices are accordingly referred to as “normally-on” devices or transistors, since the devices are effectively on under zero bias conditions. When using such normally-on transistors, provisions are generally made to ensure that a voltage is generated to make sure that the normally-on transistor may be turned off. For example, in a driver circuit used in a switch mode power supply, a negative voltage is generated or provided that has a voltage that is sufficiently below the threshold of the normally-on transistor to ensure that the device is in fact turned off as intended. Alternatively, the normally-on switch can be operated in a series connection with a normally-off switch to achieve overall normally-off behavior.